Make the destructor of WebFrameClient virtual.
Make the destructor of WebFrameClient virtual.

Most subclasses of WebFrameClient assume that the destructor of WebFrameClient
is virtual. Currently, CreateChildCounterFrameClient class in WebViewTest.cpp
is in trouble because of this.

BUG=N/A

Review URL: https://codereview.chromium.org/320583002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@175685 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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